1. Field of the Invention
The present invention relates generally to measuring devices and, more particularly, to absolute encoders for providing position information.
2. Related Art
Optical encoders are well known and employed for a variety of applications to detect position. An optical encoder measures a position of an object, either angular or linear, by optically detecting marks on a scale attached to the object to move with the object. In the simplest form, the encoder simply measures translation by counting the number of marks that move past the encoder's optical detector. One type of optical encoder is commonly known as an absolute encoder, which has the ability to recognize the position of a movable object without reference to a base, home, or starting position. In a conventional absolute encoder, each position is given by a unique code pattern of marks which identifies the absolute position, as opposed to just one mark. A change in position is sensed by detecting a change in the code bits which make up the code pattern.
In general, optical absolute rotary encoders are provided with a light source, a rotary code disk and fixed plate, and light receiving elements, such as photodetectors. A code disk typically has apertures, slits, or other transparencies distributed over the area of the disk. The angular and radial coordinates of the transparencies on the disk are known, and this two dimensional information is used to provide higher angular resolution and range than the finite physical size an individual aperture could provide. The apertures are typically arranged in concentric annular rings, with each ring having a sequence of apertures, generally at the same radius and at equal angular intervals. The code disks are fixed to a rotating element of the assembly, and the optical detectors and light sources are attached to a fixed element. Variations on this theme exercise different illumination and detection arrangements and different shapes and distributions of apertures.
In a typical operation, the portion of light from the light source passing through light passage portions provided in the rotary code disk and slits provided in the fixed plate is converted into electric signals by the photodetectors, which is decoded, for example, by referring to a look-up table, thereby allowing detection of the angle of rotation and the position of rotation. The light passage portions of the rotary code disk are comprised of a plurality of concentric code patterns and a reference pattern. The slits of the fixed plate are provided corresponding to the code patterns and reference pattern. To increase accuracy or precision, the number of light passage portions and slits are increased, which also increases the cost, complexity, and physical package size of the encoder.
Measurement applications are currently requiring higher and higher degrees of angular precision, e.g., less than a few microradians. Current high precision encoders, such as the Model BS-0001, built by Automated Precision Inc. of Gaithersburg, Md. use a compact gimbal stage and have a pointing accuracy of less than 2 microradians in each of two axes. Other types of high precision encoders, such as one built by Dr. Johannes Heidenhain, able to provide approximately 0.17 microradian resolution. However, such high precision encoders have the disadvantage of being large in size, e.g. greater than 6″ in diameter.